The present invention relates generally to printed wiring boards (PWB) and more specifically to an apparatus and method for limiting the effect of electrostatic discharges (ESD) on chips on a PWB.
Electrostatic discharge (ESD) is a problem that is prevalent in many different products and situations today. ESD is created when two different materials are brought into close proximity to each other and then separated. The process of separation causes a transfer of charges (electrons) from one material to the other.
Printed wiring boards (PWB), or circuit boards, are used in many applications, including automotive electronics such as in radios, air bag controllers, heating and air conditioning, and anti-lock brake controls. The performance of a PWB may be adversely affected by generating ESD when a PWB comes in close contact with other materials in an application. Depending on the physical circumstances, the resulting charge might flow through one or more of the electronic parts on a PWB, resulting in damage to some of the electronic parts. Engineers spend a considerable amount of time designing processes that avoid generating ESD charge, or designing circuits that direct the charge safely away from sensitive parts.
This approach has worked successfully for many years. However, as PWB boards shrink in size, and correspondingly the spaces between the materials shrink as well, different electromagnetic phenomena come into play.
It is known in the art that two charges separated by a distance have an electric field between them. That field can be described by the equation:
xe2x80x83E=V/d
where E is the electric field that results from an ESD voltage, V is the voltage (in this case ESD voltage), and d is the spacing between the points defining the voltage. The two described charges will remain separated as long as the material between the charges (assuming the material is non-conductive) retains its nonconductive properties. However, if the voltage becomes too large, or the spacing too small, the electric field can become large enough to generate a force that literally rips electrons from the non-conductive material, and a current flows. Generally speaking, this occurs when the electric field reaches levels in the b 5 MV/m range.
Not explicit in the equation is the effect of the shape of the parts on electric fields. It is well known in the art that two sharp edges will generate a much larger field between them than two blunt edges, with all else being equal, as the charge densities will be greater near the points or edges.
The current technology in the circuit board industry suggests that the size of PWBs, and correspondingly the distance between materials contained on PWBs, is increasingly growing smaller and smaller. As such, the likelihood of ESD voltages being able to jump across parts and between traces correspondingly has increased. As PWB traces typically do not have ESD protection, the potential for damage to vital components on a PWB is increased.
Thus, there is a need to provide electrostatic discharge protection of vital circuitry that accommodates the shrinking sizes of printing wiring boards.
It is, therefore, one object of the present invention to maximize the field between any trace ESD currents that might flow and the ground, thus allowing ESD energy with voltages above breakdown voltage to jump to the ground and away from any vital electronic components on the PWB.
It is another object of the present invention to adjust the impedance, which is defined as the total opposition that a circuit presents to an alternating current, between the ESD ground and the trace ESD currents on a PWB, such that an ESD breakdown voltage will jump to the ESD ground and away from vital electronic components on the PWB.
A PWB in accordance with the present invention has one or more dischargers having a size and configuration that maximizes the electric field between the discharger and any input connector pins, such that the electrostatic discharge would preferentially jump from input connector pin to discharger and be harmlessly dissipated through the ground. Such dischargers, or extensions thereof, are provided with electric field increasing features such as sharp points or edges located adjacent and directed at the input connector pins to maximize the electric field therebetween. The PWB may further have additional dischargers with similar features located in alternative locations near traces, such that electrostatic discharge would preferentially jump from traces to discharger and be harmlessly dissipated through the ground.
Other features and advantages of the present invention will become apparent from the following detailed description that should be read in conjunction with the accompanying drawings.